Crystalline polypropylene is excellent in heat resistance, chemical resistance, and electrical properties and satisfactory in stiffness, tensile strength, optical characteristics, and post-processability and has thus been used for production of various molded articles such as injection molded articles, blow molded articles, films, and sheets.
However, the crystalline polypropylene is limited in its application because the above-described properties are not always satisfactory as demanded in some applications. In particular, it is inferior to a polystyrene resin or an ABS resin in terms of stiffness properties, which has constituted a serious hindrance to increasing the application of application of the crystalline polypropylene. Accordingly, an improvement in stiffness properties would lead not only to reduction of wall thickness of molded articles which contributes to saving of resources but also to speeding up of cooling of molded articles which contributes an increase of molding rate per unit time, i.e., productivity rate.
It is conventionally known that stiffness of crystalline polypropylene may be improved by incorporating thereinto an organic nucleating agent, e.g., aluminum p-t-butylbenzoate and 1.3,2.4-dibenzylidene sorbitol, or various inorganic fillers, e.g., talc, calcium carbonate, mica, barium sulfate, asbestos, and calcium silicate.
Incorporation of an organic nucleating agent, however, not only results in considerable reduction in surface gloss, impact strength, and tensile elongation of the resulting molded articles but increase cost. Further, incorporation of the inorganic fillers not only which are advantageous characteristics of prolypropylene but also reduces surface gloss, tensile elongation and post-processability of the molded articles.
It has been proposed to obtain molded articles of high stiffness by using polypropylene having high isotacticity as disclosed in JP-A-58-104905, JP-A-58-104906, and JP-A-58-104907 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). The proposed polypropylene exhibits somewhat improved stiffness but is still unsatisfactory as compared with polystyrene or an ABS resin.